It is known that semiconductor materials, e.g. cadmium sulphide which possesses a large third order susceptibilty, have a refractive index which is dependent on the intensity of incident radiation. It has been proposed to utilise this property in the broad field of optical signal processing, e.g. as bistable elements and optical power dependent switches.
Recently a class of materials known as semiconductor doped glasses have been examined for non-linear properties. These materials consist of oxide glasses in which semiconductor crystallites are dispersed and these glasses are available as optical filters. The band gap and dimensions of the dispersed semiconductor crystallites determines the cut-off wave length of the filter. This can be varied by suitable choice of the semiconductor, and processing conditions. Ironside et al, in a paper "Wave guide fabrication in non-linear semiconductor glasses" ECOC 85 (Venice) especially at page 237, have proposed to make planar wave-guiding structures by ion-exchange with suitable glasses of this type.
Patela et al, in Electronics Letters of Apr. 10, 1986, Vol. 22, No. 8 at pages 411 and 412 consider the fabrication and properties of a non-linear wave guiding structure consisting of a thin film of Corning 7059 glass deposited on the surface of CdS.sub.x Se.sub.1-x doped glass. Abashkin et al, in Soviet Journal of Quantum Electronics, Vol. 12 (1982) October, No. 10, published in New York, USA, at pages 1343 to 1345 discuss the propagation of light in chalcogenide semiconductor fibres.
This invention comprises a glass optical fibre, e.g. a monomode fibre, wherein either the core or the cladding or both core and cladding take the form of a continuous glass phase having dispersed therein colloidal particles of a semiconductor, e.g. a semiconductor having a band gap of 3.8 to 0.27 ev, especially 2.5 to 1.4 ev. The colloidal particles preferably have sizes in the range 1 to 1000 nm.
Examples of suitable semiconductors include
______________________________________ CdS.sub.w Se.sub.(1-w), CdSe.sub.x Te.sub.(1-x), PbS.sub.y Se.sub.(1-y), CuCl, CuBr, ZnS.sub.a Se.sub.(1-a), Pb.sub.b Cd.sub.(1-b) Se, In.sub.2 Te.sub.3 Sb.sub.2 Se.sub.3 ______________________________________
wherein w, x, y, a and b are 0 to 1 inclusive.
The invention also comprises devices which include a fibre according to the invention. Such devices preferably comprise an un-jointed length of fibre having at least one active region and at least one passive region wherein, in the or each passive region, the semiconductor is dissolved in the glass continuous phase and, in the or each active region, the semiconductor is colloidally dispersed. This preferred form confines the non-linear activity to the region(s) where it is desired, i.e. to the active region(s) where the particles are present. It also avoids losses because there are no joints between the active region(s) and the feeder(s).
The fibres according to the invention are fabricated by the conventional drawing of a preform which contains the semiconductor either dispersed or dissolved. If the semiconductor is dissolved, or if it dissolves during processing, then it is necessary to precipitate it as a colloid in the final fibre. This precipitation may be achieved by:
(a) heat treatment, e.g. relatively short periods at high temperature or longer periods at low temperatures such as 1 minute at 700.degree. C. or 30 minutes at 600.degree. C.; PA1 (b) laser initiation at wave lengths where the host glass absorbs, e.g. UV and IR; PA1 (c) electron beam treatment; PA1 (d) ion bombardment.
When the final product contains active and passive regions the treatment is applied only to the active region or regions.
Some of the semiconductor dopants which are used in the invention are well known as colourants for glass and, in particular, as additives for filter glasses which have a well defined cut-off wave length. (Some semiconductors, e.g. those having band gaps of 0.27 to 0.4 ev such as PbSe and PbS have cut-offs at wavelengths in the region of 3 .mu.m and these appear black to the eye).